A thermocouple is really a widely used form of sensor that is utilized to measure temperature. Thermocouples are popular in industrial control applications due to their relatively low priced and wide measurement ranges. Specifically, thermocouples excel at measuring high temperatures where other common sensor types cannot function. Try operating an internal circuit (LM35, AD 590, etc.) at 800C.
Thermocouples are fabricated from two electrical conductors made from two different metal alloys. The conductors are normally that are part of a cable using a heat-resistant sheath, often with the integral shield conductor. At one end of your cable, the two conductors are electrically shorted together by crimping, welding, etc. This end of your thermocouple–the hot junction–is thermally linked to the object to get measured. Other end–the cold junction, sometimes called reference junction–is connected to a measurement system. The goal, needless to say, is to ascertain the temperature nearby the hot junction.
It needs to be noted that this “hot” junction, that is somewhat of the misnomer, may actually attend a temperature lower compared to the reference junction if low temperatures are being measured.
Since thermocouple voltage is really a function of the temperature distinction between junctions, it is essential to know both voltage and reference junction temperature as a way to determine the temperature in the hot junction. Consequently, a thermocouple measurement system must either measure the reference junction temperature or control it to keep it at the fixed, known temperature.
You will discover a misconception of how thermocouples operate. The misconception is that the hot junction will be the supply of the output voltage. This can be wrong. The voltage is generated across the duration of the wire. Hence, if the entire wire length are at exactly the same temperature no voltage can be generated. If it were not true we connect a resistive load to a uniformly heated temperature sensor thermocouple inside an oven and make use of additional heat through the resistor to create a perpetual motion machine of your first kind.
The erroneous model also claims that junction voltages are generated at the cold end between the special thermocouple wire and also the copper circuit, hence, a cold junction temperature measurement is required. This idea is wrong. The cold -end temperature will be the reference point for measuring the temperature difference across the length of the thermocouple circuit.
Most industrial thermocouple measurement systems prefer to measure, as an alternative to control, the reference junction temperature. This is certainly due to the fact that it is more often than not less expensive just to put in a reference junction sensor to an existing measurement system rather than add-on a whole-blown temperature controller.
Sensoray Smart A/D’s study the thermocouple reference junction temperature by means of a dedicated analog input channel. Dedicating a unique channel to this particular function serves two purposes: no application channels are consumed with the reference junction sensor, as well as the dedicated channel is automatically pre-configured with this function without requiring host processor support. This special channel is made for direct connection to the reference junction sensor that may be standard on many Sensoray termination boards.
Linearization Within the “useable” temperature selection of any thermocouple, you will discover a proportional relationship between thermocouple voltage and temperature. This relationship, however, is in no way a linear relationship. In reality, most thermocouples are really non-linear over their operating ranges. To be able to obtain temperature data coming from a thermocouple, it really is essential to convert the non-linear thermocouple voltage to temperature units. This thermocoup1er is referred to as “linearization.”
Several methods are normally utilized to linearize thermocouples. With the low-cost end of your solution spectrum, one can restrict thermocouple operating range in a way that the thermocouple is nearly linear to in the measurement resolution. In the opposite end of your spectrum, special thermocouple interface components (integrated circuits or modules) are offered to perform both linearization and reference junction compensation within the analog domain. Generally, neither of such methods is well-designed for inexpensive, multipoint data acquisition systems.
In addition to linearizing thermocouples inside the analog domain, it is easy to perform such linearizations from the digital domain. This really is accomplished by using either piecewise linear approximations (using look-up tables) or arithmetic approximations, or sometimes a hybrid of the two methods.
The Linearization Process Sensoray’s Smart A/D’s hire a thermocouple measurement and linearization method that was created to hold costs to your practical level without sacrificing performance.
First, both the thermocouple and reference junction sensor signals are digitized to acquire thermocouple voltage Vt and reference junction temperature Tref. The thermocouple signal is digitized in a higher rate than the reference junction because it is assumed that this reference junction is fairly stable compared to the hot junction. Reference junction measurements are transparently interleaved between thermocouple measurements without host processor intervention.